Adoboli, 35, was freed from Maidstone Prison in southeast England last week, having served half of his sentence, the source close to the investigation said.

The Ghanaian-born banker was convicted to seven years for fraud by abuse of position for losing the Swiss bank's money, and to four years for a second count of the same offence, which he was to serve concurrently.

He was sentenced in November 2012 but had already been detained for nine months prior to his trial.

Adoboli is understood to be staying with friends while he attempts to put his life back together.

During Adoboli's trial, he claimed senior managers were fully aware of his activities and encouraged him to take risks to make profits for Switzerland's biggest bank.

But prosecutors said that in a bid to boost his bonuses and chances of promotion, Adoboli exceeded his trading limits, failed to hedge trades and faked records to cover his tracks between 2008 and 2011.

The tactics initially paid off, but as the financial crisis took hold, Adoboli's deals went bad.

The court was told that at one point the privately-educated son of a former United Nations official was at risk of causing UBS losses of $12 billion.

His case drew comparisons to Jerome Kerviel, the French trader who lost the Société Générale bank 4.9 billion euros ($5.5 billion) in 2008, and British rogue trader Nick Leeson, who caused the collapse of Barings Bank in 1995.

Adoboli's arrest in September 2011 wiped ten percent off UBS's share price.

He had sent an email in which he confessed that UBS was exposed to colossal losses resulting from his unauthorized trades.

During his trial, the court heard that Adoboli began conducting off-the-books trades in 2008, holding them off the ledger until the market rebounded.

However, he racked up huge losses and discrepancies in his trading activities eventually aroused suspicions.

Adoboli said UBS was “everything I lived for,” and stated he was only trying to generate profits for the bank as the global financial crisis plunged the markets into turmoil.

He said more senior traders quit the business as the crisis took hold, leaving “kids” like himself in charge of keeping a $50-billion portfolio afloat.

It might seem “crazy” that traders with just a few years' experience were in charge of such a huge portfolio, Adoboli told the jury, but “that's how it was.”